The long term goal of the proposed research is to understand how cellular or external stimuli achieve specific nuclear depletion of a regulatory molecule and thus control eukaryotic cell fate, by using Arabidopsis as an experimental system. Previous studies have demonstrated that Arabidopsis developmental regulator, COP1, acts in the nucleus in the dark to assert its repressive effects, while light stimuli triggers the nuclear depletion of COP1 thus abrogating its action. Several putatively involved cellular components have been identified by genetic and biochemical means, and one of them, COP9, has been shown to be a component of the nuclear envelope. My specific objectives during the proposed training period are to determine the precise subcellular location of COP9 and others by available cytological techniques using both electron and light microscopy, and to unequivocally demonstrate which of these components are directly involved in the light control of COPI nucleocytoplasmic trafficking by a combination of cell biological and molecular genetic means. Accomplishment of these objectives will provide unprecedented insights into the mechanisms of stimulus-responsive macromolecular trafficking across the nuclear envelope in eukaryotic cells and add to our general understanding of the basis of human diseases.